1. Field of the Invention
This invention relates to an ophthalmic apparatus in which functions, such as measuring the shape of the cornea of an eye to be examined and determining the accuracy of alignment, are provided to an observation device having an illuminating light source for observing the front eye part of the eye to be examined, such as a microscope for surgical operation.
2. Related Background Art
An apparatus in which a microscope for surgical operation is provided with the cornea measuring function is known as an example of an apparatus in which the measuring function is provided to an observation device. This apparatus is used to accurately control the degree of cornea astigmatism or accurately measure the refractive power of the cornea during cornea suturing performed during, for example, a surgical operation for removing a cataract, thereby providing good eyesight after surgical operation. In such an apparatus, there has heretofore been proposed a technique whereby an index mark for measurement, referred to as so-called Mire image having a predetermined shape, for example, a circular slit-like shape, is projected onto the cornea so that the shape of the corneal reflection image thereof can be detected and measured by an electrical sensor.
In this case, however, the illuminating light for observation by the microscope during a surgical operation is very intense and therefore, even if the aforementioned index mark for measurement is projected onto the cornea, the optical information of the corneal reflection image of the projected index mark is affected by the corneal reflection image of the illuminating light source for observation, and it is very difficult to electrically detect such optical information. That is, the cornea has a convex mirror action of strong reflective power, and when the Mire image M for measurement is to be detected as shown in Figure 1 of the accompanying drawings, it is difficult to obtain a signal of good S/N ratio due to the filament image F of an illuminating lamp for observation by the corneal reflection even if the light source for the Mire image M is made intense. In FIG. 1, l.sub.1, l.sub.2 and l.sub.3 designate the directions of measurement for measuring the positions of the Mire image M in three meridian directions.